ESR1 and breast carcinoma: It is well-known that the overexpression of estrogen receptor α(ERα) may lead to ER-positive breast cancer, which accounts for 70% of breast cancer.63,64 Approved endocrine therapies include aromatase inhibitors(AIs) such as Letrozole, selective ER modulators (SERMs) such as Tamoxifen, and selective ER degrader (SERDs) such as Fulvestrant.